Hi Erwin,

You are right to take a careful approach to those GM-style AC fittings. I haven't broken one yet, so I really don't know how to tell you what not to do.

First I try to be sure the tubing enters the nut squarely. Any angle or stress toward the side will add to binding forces. Then gentle working back and forth, holding the tubing with a bare hand, might break up the white powdery corrosion. Tapping the nut with a very light ball pein hammer helps if powder falls out of the joint. I'm not sure how much PB'laster helps to get it moving, but once it does move, it helps things continue to move. I haven't resorted to heat, and would think it a very last resort, and if you do and see any green flame*, stop at once, hold your breath, and get away. (internet disclaimer)

*Flame-type leak detectors for Freon use the bright green of poisonous phosgene gas as an indicator. AFAIK this works for R-134a as well as it did for R-12.
--
Art Benstein near Baltimore

The average dog is a nicer person than the average person.
-Andy Rooney